判斷ffmpeg讀到的packet是否爲關鍵幀
av_read_frame(avFormatCtx, readPacket)
if(readpacket->flags &AV_PKT_FLAG_KEY )
{
key frame
}
//////如果是h264編碼，從幀數據判斷
frame = |length|data|
length後面緊跟着的第一個byte後5位如果是01001則爲關鍵幀
if((frame[4] & 0x1f) == 5)
{
NSLog(@"關鍵幀");
}else{
NSLog(@"非關鍵幀");
}